Silicon tetrafluoride is widely used in the manufacturer of semi-conductor chips, pyrogenic silica, and other industrially important chemicals.
Silicon tetrafluoride can be produced in several ways all of which are based on reacting silica (SiO.sub.2) with either hydrofluoric acid (HF) or fluorosilicic acid (H.sub.2 SiF.sub.6). Thus to produce SiF.sub.4 from SiO.sub.2, the production of either hydrofluoric or fluorosilicic acid intermediate is required.
Current processes to produce SiF.sub.4, in varying degrees of purity, include the reaction of silica with hydrogen fluoride gas according to the reaction: EQU SiO.sub.2 (s)+4HF(g).fwdarw.SiF.sub.4 (g)+2H.sub.2 O
See U.S. Pat. No. 4,382,071.
Purity of the SiF.sub.4 is dependent on the source of the silica and hydrogen fluoride reagents. The reaction is typically carried out at 25.degree.-55.degree. C. in concentrated sulfuric acid (&gt;80% H.sub.2 SO.sub.4) in order to diminish the reverse reaction through capture of the product H.sub.2 O. This process also uses large amounts of HF which raises concerns for corrosion, safety and environmental management.
Production of SiF.sub.4 from fluorosilicic acid can be accomplished according to the reaction: EQU SiO.sub.2 (s)+2H.sub.2 SiF.sub.6 (aq).fwdarw.3SiF.sub.4 (g)+2H.sub.2 O
See U.S. Pat. No. 4,470,959. This reaction is also carried out in concentrated sulfuric acid (&gt;80% H.sub.2 SO.sub.4), but usually at a slightly higher temperature, between 25.degree.-95.degree. C.
It is also possible to produce SiF.sub.4 directly from fluorosilicic acid by thermal decomposition: EQU H.sub.2 SiF.sub.6 (aq).fwdarw.SiF.sub.4 (g)+2HF(g)
However, typical input fluorosilicic acid (20-30% aqueous) comes from fertilizer and phosphoric acid/super phosphate manufacturing waste tails. The fluorosilicic acid is generally low grade containing many impurities such as phosphorous, nitrogen and sulfur, all of which are detrimental to producing high purity SiF.sub.4.
Yet another multi-step process for producing SiF.sub.4 utilizes the reaction of fluorosilicic acid with sodium fluoride and silicon dioxide according to the reaction: EQU 2H.sub.2 SiF.sub.6 (aq)+6 NaF+SiO.sub.2 .fwdarw.3Na.sub.2 SiF.sub.6 (s)+2H.sub.2 O
followed by thermal treatment of the fluorosilicate salt at 600.degree. C. to release SiF.sub.4 according to the reaction: EQU Na.sub.2 SiF.sub.6 .fwdarw.SiF.sub.4 (g)+2NaF
See U.S. Pat. No. 4,615,872.
As with the processes discussed above, this introduces impurities through use of low grade fluorosilicic acid diminishing the purity of the silicon tetrafluoride produced.
However, uranium is a naturally occurring element which is comprised of approximately 0.7%.sup.235 U and 99.3%.sup.238 U. .sup.235 U is used to produce Nuclear Energy, while .sup.238 U is not. Because of the low percentage of .sup.235 U found in naturally occurring uranium, naturally occurring uranium must be enriched in order to obtain sufficient amounts of .sup.235 U which will support nuclear fission. This enrichment process, aside from producing high concentrations of .sup.235 U, produces huge amounts of depleted uranium hexafluoride (UF.sub.6 by-product which is a very hazardous compound posing a serious health threat. Since depleted uranium metal is radioactive and expensive to produce, it is used in limited quantities for highly specialized applications. Accordingly, alternative uses are needed in order to avoid having to dispose of the UF.sub.6 at great expense by storing it indefinitely.
One solution to reducing the large stores of UF.sub.6 is to reduce UF.sub.6 to UO.sub.2 F.sub.2 and convert the UO.sub.2 F.sub.2 into SiF.sub.4, and an oxide of uranium, e.g. UO.sub.2,UO.sub.3 or U.sub.3 O.sub.8.
One use for uranium oxide is to add it to the concrete which is used to build bunkers in which radioactive waste is stored in order to provide high density shielding for the stored high level radioactive waste. Uranium oxide possesses outstanding radioactive shielding properties, and when added to concrete in the form of aggregate makes a very effective, low cost shielding material.
Thus, the use of UO.sub.2 F.sub.2 as the fluorine source for production of silicon tetrafluoride eliminates the problems of impurities introduced by the use of fluorosilicic acid. UO.sub.2 F.sub.2 is a relatively inert solid that is easy to handle with proper precaution for containment of low level radioactive materials. UO.sub.2 F.sub.2 is also produced in very high purity, being derived from the highly refined UF.sub.6. Thus, the use of UO.sub.2 F.sub.2 has both technical and economic advantages in the production of high purity SiF.sub.4.